S100B is a member of the S100 protein family. S100 protein is a low molecular weight protein found in vertebrates characterized by two calcium binding sites of the helix-loop-helix (“EF-hand type”) conformation. There are at least 21 different types of S100 proteins. The name is derived from the fact that the protein is 100% soluble in ammonium sulfate at neutral pH. S100B is an acidic protein with a molecular weight of 21 kDa existing as a homodimer consisting of two beta subunits. The two monomers are configured in a twofold axis of rotation and are held together by disulfide bonds. S100B is involved in the regulation of energy metabolism in brain cells. S100B is produced primarily by astrocytes and exerts autocrine and paracrine effects on glia, neurons, and microglia. Furthermore, it interacts with many immunological functions of the brain (reviewed in Rothermundt et al. 2003).
Functional S100B promoter sequences have been identified and analyzed in several mammalian species. The structure of the human S100B gene and proximal promoter region was initially elucidated by Allore (1990). Castets et al. (1997) analyzed several human constructs containing different fragments of the human S100B promoter which revealed a complex pattern of regulation relating to different regions of the promoter. In this study, a number of positive and negative regulatory elements that are at least partially involved in regulating cell specific expression were identified in the region between −1012 and +697 (with +1 being defined as the first nucleotide in exon 1 of the S100B gene). Additionally, a negative regulatory element is thought to be located in the region between −4437 and −1012 of the upstream region, however the precise location of this element was not elucidated. This study highlighted the complexity of the regulation of this gene to allow specific cell specific expression of human S100B. Lin et al. (2004) analyzed p53 transcription factor binding sites and their effect on human S100B promoter activity in malignant melanoma cells.
Expression of S100B has been shown to be spatiotemporally associated with maturation of glial cells in mice (Deloulme et al. 2004; Raponi et al. 2007) and rats (Hagiwara and Sueoka 1995). In a separate study, an enhanced green fluorescent protein (EGFP) reporter was fused to the −1669/+3116 region of the mouse S100B gene, resulting in observed expression in both astrocytes and oligodendrocytes, an expression pattern that occurred in both a spatial and temporal fashion during mouse brain development (Vives et al. 2003). More recent data using this same expression construct further highlights the spatiotemporal expression pattern in both astrocytic and oligodendrocytic lineages in the mouse brain (Hachem et al. 2005). Analysis of expression patterns in humans have also revealed spatiotemporal expression during brain development, particularly in proliferating and mature astrocytes (Marshak 1990; Tiu et al. 2000). Antibodies raised to S100B have been shown to label primarily astrocytes in human brain (Lyck et al. 2008).
There exists a significant need for promoter elements which are capable of driving expression in specific cell types and/or in specific regions of the brain. Identification of minimal elements required for adequate expression and specificity will allow ease of use in expression constructs.